In recent times there has been a great focus of attention on the study of proteases, since protease inhibitors have been shown to be effective in extending the life of people with AIDS. We have been studying chymotrypsin, a protease enzyme that was previously believed to be synthesized only in the pancreas. We have generated a rabbit antiserum to human chymotrypsin and have found chymotrypsin to be present in endocrine-like cells of the villi in the stomach and in epithelial cells of the intestine. By a ribonuclease protection assay we also detected chymotrypsin mRNA in the rat stomach, duodenum, ovary and spleen. We have also identified chymotrypsin mRNA in the rat and human brain. Electrophoresis and immunoblotting revealed chymotrypsin bands in the stomach, duodenum and brain. Synthesis of a potent protease such as chymotrypsin in tissues other than pancreas is significant, suggesting a potential physiological and/or pathological role in these tissues.